Basically, Hereness is the quality of being present and living in the moment, and Thereness is when your mind wanders and explores the distant or recent past; the immediate or far future. Daydreams, memories, plans. Anywhere but here. And anytime but now.
Hereness is simple. It is only this moment, nothing else. It is awareness and appreciation of your immediate surroundings; often through the senses. It is real.
Thereness is more complex; it can be many things. It can be idealistic, used as a distraction to avoid thinking or doing something perceived as unpleasant. It can be dwelling on a positive (or negative) memory. It can be running through all the to-do items on your list. For me, most of the time, Thereness tends to the Daydream end of the spectrum, and is a yearning, a wanting to be There: Away. Where things are different, better somehow. Warmer. Brighter. Rose-coloured.